The Molten Core is a level 60 raid dungeon located within Blackrock Mountain, home to Ragnaros the Firelord. It was one of two forty (40) player raid dungeons available upon release of World of Warcraft, with the other being Onyxia's Lair. Considered the pinnacle of raiding achievement at the time, the Molten Core enticed adventurers to enter the instance in the search of realm glory.
Just before the re-opening of the Dark Portal, Ragnaros, the land's fiery overlord was cast down from his dark throne and returned to the Firelands. He now rages war on the world tree Nordrassil.

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Upon successful completion of the encounters, all bosses drop tier 1 set items - excluding Ragnaros, who drops tier 2 leg items. In addition, trash mobs may drop Fiery Cores, Lava Cores, and Core Leather, which can be used to craft armor and weapons available with Thorium Brotherhood reputation.

The Hydraxian Waterlords offer a quest chain, eventually rewarding Aqual Quintessence, which was at one point required to douse the Runes of the Firelord in order to summon Majordomo Executus and consequently, Ragnaros. Presently, defeating bosses will automatically douse their respective runes.

The Molten Core was created during the War of the Three Hammers more than 300 years ago. Near the end of the war, Thaurissan, the leader of the Dark Iron Dwarves, sought to summon a powerful fire elemental to defeat the combined forces of the Bronzebeard and Wildhammer clans. He was more successful than he could have imagined, as he released Ragnaros the Firelord from millennia of captivity under the Redridge Mountains.

Ragnaros destroyed the city of Thaurissan and created the volcano of Blackrock Mountain. He dwells there to this day with his elemental servants and the enslaved remnants of the Dark Iron dwarf clan. The burning lake where Ragnaros lies sleeping acts as a rift connecting to the plane of fire, allowing the malicious elementals to pass through.

From old World Dungeons site:

The Molten Core lies at the very bottom of Blackrock Depths. It is the heart of Blackrock Mountain and the exact spot where, long ago in a desperate bid to turn the tide of the dwarven civil war, Emperor Thaurissan summoned the elemental Firelord, Ragnaros, into the world. Though the firelord is incapable of straying far from the blazing Core, it is believed that his elemental minions command the Dark Iron dwarves, who are in the midst of creating armies out of living stone. The burning lake where Ragnaros lies sleeping acts as a rift connecting to the plane of fire, allowing the malicious elementals to pass through. Chief among Ragnaros' agents is Majordomo Executus - for this cunning elemental is the only one capable of calling the Firelord from his slumber.

On release, Molten Core and Onyxia's Lair were the only 40 man instances in the game, thus Ragnaros was the first "game end boss." With the advent of Blackwing Lair, Ahn'Qiraj, Naxxramas, and the raising of the level cap with the game's expansions, defeating Ragnaros has long since ceased to be the pinnacle of raiding success. However, prior to the Burning Crusade, slaying him was still considered a very important step in raid progression before engaging in more challenging content. It was the first raid encounter which required a concerted effort in acquiring massive amounts of a particular statistic (in this case, fire resistance). Moreover, the encounter was the first one to demand the extensive use of consumables, at least in the learning process and in the first few kills. As such, it was once a very important stage in the evolution of a raiding guild.

Some of Ragnaros' minor minions waiting to welcome you into the Molten Core.

While technically there are no prerequisites to enter Molten Core through its entrance at the bottom of Blackrock Depths, every Core raider will want to complete the Attunement to the Core quest to enable the shortcut at the bottom of Blackrock Mountain. This quest is given at level 55 by Lothos Riftwaker who is located just outside The Quarry, on the way to Blackrock Depths. Once attuned you can access Molten Core by talking to Lothos again. When dead (as ghost) you also have the possibility to jump into the lava near Lothos. This can also be done by living players if they have a way to slow fall, or by jumping first onto the small ledge in the pillar on the right as you enter the main chamber of Blackrock Mountain, healing, then jumping down, back toward the wall of the lava pit, to cover the remaining distance to the platform below.

There are very few quests that involve the Molten Core. The only significant quest chain is the Hydraxian Waterlords chain, which can be started by a visit to Duke Hydraxis off the coast of Azshara. Completing this quest chain will require killing one of each of the trash mobs in the Core, and then defeating the four Flamewaker bosses in Molten Core (Lucifron, Gehennas, Shazzrah, and Sulfuron Harbinger). In addition, all the trash mobs and bosses in Molten Core provide Hydraxian Waterlord reputation. The final reward options for this quest chain are [Tidal Loop] or [Ocean's Breeze], both rare rings with high Fire Resistance.

Before Ragnaros can be summoned, the raid must defeat Majordomo Executus. Majordomo Executus will only appear if the raid douses the seven runes that are located near certain bosses in the Molten Core. Prior to Patch 3.0.8, the runes could only be doused with the use of Aqual Quintessence (which can only be acquired by speaking with Duke Hydraxis), or via the use of Eternal Quintessence (usable once per hour, and which can be obtained from Duke Hydraxis when a player reaches Revered status with the Hydraxian Waterlords). With the introduction of Patch 3.0.8, killing the boss that is associated with a rune will now automatically douse the rune.

Molten Core is a raid instance. This means that it is on a strict timer schedule, which resets once a week, and once a character has been "saved to the instance" by killing a boss, that character cannot enter a different instance of Molten Core that week.

Each of the bosses in Molten Core has a very specific loot table, which includes the Tier 1 armor pieces as well as a number of other epic-quality items. In addition, the trash mobs in Molten Core provide Fiery Cores, Lava Cores, and Core Leather, which can be used with recipes earned at high levels of Thorium Brotherhood reputation to craft epic armor and weapons. It is generally the case that a raid progressing through Molten Core will use these Cores to craft Fire Resistance armor, which will allow the melee DPS of the raid to survive the final encounters with Majordomo Executus and Ragnaros.

With the arrival of World of Warcraft: The Burning Crusade the popularity of the Molten Core took a turn for the worse, with items available in the expansion superior in quality to that of the Molten Core or any of the other forty player instances at the time. However, frustration with the hours-long instance duration were alleviated, allowing players to power through the Molten Core in record time - trivializing content. New abilities and better gear reduced the numbers of players needed to complete Molten Core from 40 to below 10.

In Wrath Of the Lich King, it is possible for some classes to solo Molten Core. The classes doing this are usually Paladins and Death Knights, but Hunters, Shamans, Warlocks and Warriors have also managed to single-handedly defeat bosses.

Molten Core boss creatures have all received some new items on their loot tables.

Most of the non-class set items that dropped from Molten Core have had their effective levels increased, resulting in higher DPS weapons, and/or better stats and effects.

The loot tables for some of the Molten Core bosses have been improved so they drop an additional epic item per kill.

Prior to this patch, Molten Core boss creatures could drop items from one of two different tiered sets for every character class. The loot tables have been changed so Molten Core bosses will now only drop items from the lower tier class sets. However, some of the items from those sets were not previously dropping in Molten Core. These missing class set items will now drop in Molten Core, allowing players to complete their lower tier class sets.